This invention relates to a shift control for an automatic transmission, and more particularly to a driver responsive control of a power-on downshift.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
Various techniques have been used for electronically controlling the on-coming and off-going clutches during a power-on downshift. For example, the U.S. Pat. Nos. 5,029,494 and 5,070,747 to Lentz et al. disclose power-on downshift controls in which the off-going clutch is controllably released in an established sequence to allow the engine to accelerate the transmission input shaft to a target speed, whereafter the shift is completed by engaging the on-coming clutch and disengaging the off-going clutch. Similar control techniques are also described in the U.S. Pat. No. 4,653,351 to Downs et al., and the U.S. Pat. Nos. 4,796,490 and 5,079,970 to Butts et al.
In addition to defining the various control sequences of a shift, certain parameters must be selected or calibrated for controlling the rate of shift progression in order to achieve a high quality shiftxe2x80x94that is, a shift that feels smooth without creating durability concerns or excessive heating due to clutch slippage. In Lentz et al. U.S. Pat. No. 5,070,747, for example, the off-going clutch pressure is controlled to make the transmission input speed follow an established input speed trajectory or profile. In Downs et al. U.S. Pat. No. 4,653,351, on the other hand, the off-going pressure is progressively released until an established amount off-going clutch slippage occurs, and then maintained at that value until synchronization occurs. While these controls are designed to produce high quality shifting, experience has shown that under certain conditions, such as when passing another vehicle, fast response is more important than shift smoothness. Accordingly, what is needed is a driver responsive shift control that provides high quality shifting under ordinary conditions and highly responsive shifting under high demand conditions.
The present invention is directed to an improved power-on downshift control wherein the off-going clutch pressure is controlled to achieve a desired trajectory of the input shaft during the shift, and wherein the desired trajectory is scheduled to optimize shift smoothness under ordinary conditions and shift responsiveness under high driver demand conditions. A driver demand indication such as engine throttle position is monitored, and when its rate of increase exceeds a reference rate, a high rate condition is identified and the current demand indication is stored for future reference. A high demand indication or flag is set or cleared based on the high rate indication and a comparison of the driver demand indication to the stored value. The downshift trajectory is determined in dependence on the high demand indication, and is set to a first calibrated value for achieving a smooth downshift when the high demand indication is cleared, and to a second calibrated value for achieving a quick and firm downshift when the high demand indication is set. In a preferred embodiment, other shift parameters including the initial off-going release rate and the synchronization hold interval are also adjusted in dependence on the high demand indication.